Mixtures or blends of alkali-soluble resins with alkali-insoluble emulsion polymers have been used in inks and floor polishes. The alkali-soluble resins were generally prepared by solution polymerization, such as described in U.S. Pat. No. 3,037,952. Significant improvements in stability, water resistance, rheology, and efficiency were achieved by polymerizing one component in the presence of the other to form core-shell polymers as described in pending U.S. patent application of Albert B. Brown, et al., Ser. No. 872,714, filed June 10, 1986. This one-pot technique of Brown, et al., produced core-shell polymers having a physical interaction between the two polymer phases.
While the core-shell polymers of Brown, et al., represented significant improvements over the mixtures or blends of the prior art, further improvements in stability were desired. Specifically the use of certain additives, especially those containing organic solvent, caused the prior art blends and the core-shell polymer compositions of Brown, et al., to become unstable and thus unsuitable for many applications. For example, isopropyl alcohol is added to make inks and overvarnishes in the graphic arts field. Thus stability toward alcohols and other organic solvents is an essential requirement for the neutralized core-shell polymers to be used in this area.
The present invention eliminates the problems associated with solvent instability of prior art blends and core-shell polymer compositions by providing core-shell polymers wherein the core and shell components are chemically grafted together to a significant or substantial extent. The grafted core-shell polymers of this invention remains permanently attached in both aqueous and non-aqueous solvent. Additionally the grafted core-shell polymers offer the advantages of improved rheology, redispersability of the latex core, decreased foaming, less crawling or dewetting during recoating, and greater flexibility during synthesis in the selection of monomers and process additives.
The present invention involves preparation of the grafted core-shell polymers via an aqueous-based emulsion polymerization of both the core and shell. U.S. Pat. Nos. 4,465,803 and 4,565,839 disclose aqueous emulsion compositions prepared by emulsion polymerization of radical-polymerizable monomers in the presence of a water-soluble resin having a graft-polymerizable unsaturated group in the side chain. The '803 and '839 patents are distinguishable from the present invention in several respects. Firstly, the '803 and '839 patents teach initial preparation of the water-soluble resin by solution polymerization in organic solvents such as cellosolve-type, carbitol-type, and alcohol-type solvents, followed by emulsion polymerization at a high pH of radical-polymerizable monomers in the presence of the water-solubilized resin. In contrast, applicant's invention involves a completely emulsion polymerization in an aqueous medium at a low pH (i.e., less than 6). Applicant's aqueous emulsion polymerization of the alkali/water-soluble resin gives cost, efficiency and safety advantages over the solution polymerization technique taught in the '803 and '839 patents. The use of organic solvents in solution polymerization presents a safety and environmental hazard due to the flammable and toxic nature of the solvents. Additionally, solution polymerization requires expensive stripping of the solvent from the resin to produce an aqueous-based composition. Secondly, emulsion polymerization according to applicant's invention results in improved efficiency because a higher concentration of resin can be produced in the low pH process. Thus, applicant's emulsion polymerization process can result in a 45-50% by weight resin solids concentrations, whereas the solution polymerization process is typically limited to a resin concentration of about 30-38% because of the greater viscosity.
Thirdly, applicant's emulsion polymerization invention also results in better particle size control and a more narrow particle size distribution than with the solution polymerization process.
Fourthly, applicant's invention utilizes polyfunctional compounds which include unsaturated graft sites and those containing abstractable atoms. These graft sites can be incorporated into either the alkali-insoluble core or the alkali-soluble shell of applicant's core-shell polymers. However, the '803 and '839 patents only disclose unsaturated graft sites in the water-soluble resin component.
Fifthly, applicant's preferred embodiment involves emulsion polymerization of the alkali-insoluble core first, followed by emulsion polymerization of the alkali-soluble shell. This results in a more crosslinked core which can have a greater stability toward alcohol/solvents and other additives than the polymers prepared in the '803 and '839 patents. A crosslinked core can also result in enhanced film properties, such as block resistance.